U.S. Pat. No. 7,002,613 describes a digital printing system to which the imaging device of the present disclosure is applicable, by way of example. In particular, in FIG. 8 of the latter patent specification, there is shown an imaging device designated 84 that is believed to represent the closest prior art to the present disclosure. The imaging device serves to project a plurality of individually controllable laser beams onto a surface, herein termed an imaging surface, to generate an energy image onto that surface. The laser image can be used for a variety of purposes, just a few examples being to produce a two dimensional printed image on a substrate, as taught for instance in U.S. Pat. No. 7,002,613, in 3D printing and in etching of an image onto any surface.
For high throughput applications, such as commercial printing or 3D lithography, the number of pixels to be imaged every second is very high, demanding parallelism in the imaging device. The laser imaging device of the present disclosure is intended for applications that require energy beams of high power where the total power required can be of tens or hundreds of milliwatt (mW). For instance, in the field of printing, depending on the desired printing speed, the energy beams can provide powers of up to 10 mW, 100 mW and even 250 mW or higher. One cannot therefore merely scan the imaging surface with a single laser beam, so as to expose the pixels sequentially. Instead, the imaging device is required to have a plurality of laser emitting elements for various pixels (picture elements) each laser capable of tracing a line of pixels in the image area of an imaging surface in relative motion.
To achieve acceptable print quality, it is important to have as high a pixel density as possible. A high resolution image, for example one having 1200 dpi (dots per inch), requires a density of laser emitting elements that is not achievable if the laser emitting elements all lie in a straight line, due to the amount of overlap necessary between the laser sources to achieve a uniform printing quality. Aside from the fact that it is not physically possible to achieve such a high packing density, adjacent elements would interfere thermally with one another.
Semiconductor chips are known that emit beams of laser light in an array of M rows and N columns. In U.S. Pat. No. 7,002,613 the rows and columns are exactly perpendicular to each other but the chips are mounted askew, in the manner shown in FIG. 1 of the latter patent, so that each row can fill in the missing pixels of the preceding row(s). In this way, such an array can achieve a high resolution image but only over the width of the chip and such chips cannot simply be mounted side by side if one is to achieve a printed image without stripes along its length, because the chips cannot have laser emitting elements positioned sufficiently close to their lateral edges.
U.S. Pat. No. 7,002,613 avoids this problem by arranging such chips in two rows, in the manner shown in FIG. 8 of the latter patent. The chips in each row are staggered relative to the chips in the other row of the pair so that each chip in one row scans the gap left unscanned by the two adjacent chips in the other row.
Even though it is expected that the rows of chips will be mounted on a support under clean laboratory conditions using a microscope to achieve their correct alignment, it is guaranteeing that the relative alignment of the chips in the two rows will be accurate within the resolution of the printed image is difficult and expensive. Any misalignment will result in the image having stripes or other undesired defects.
US 2010/080594 and US 2008/181667 describe systems in which the light from arrays of LED's (rather than laser sources) is projected onto an image surface and teach how steps may be taken to compensate for any misalignment between the arrays. In each case, the images produced by adjacent arrays are overlapped and selected LED's from one or other of the two arrays are activated to maintain image continuity at the boundary between the two arrays. In the case of US 2010/080594 this overlap is shown clearly in FIG. 14 and in US 2009/181667 it is evident, for example, from FIGS. 9A and 9B.